1. Field of the Invention
The present invention relates to a process for oxidizing an organic material, for example, a resist such as a photoresist, X-ray resist, electron beam resist, to be used in the production of, for example, semiconductor devices, to thereby effect an ashing thereof, as well as in the production of liquid crystal devices and plasma displays.
In the production of semiconductor devices, usually a photoresist (hereinafter abbreviated as resist) is used as a mask when etching a wafer, and as the resist becomes unnecessary after the etching treatment, it must be removed. The removal methods include the wet treatment system in which a resist stripping solution is used, and the dry treatment system of ashing the resist in an active species of oxygen atoms or an oxygen plasma.
Currently, dry treatments, which comprise simple ashing steps but can also treat the resist carbonized in the process of ion injection, are widely used, and among these treatments, the downstream ashing method, which causes little or no damage to the wafer, is most widely used.
In the currently used dry treatment utilizing the downstream ashing method, the method in which a gas composed mainly of oxygen gas is used for the ashing is usually employed. Nevertheless, since the ashing rate (i.e., amount of organic material per unit of time) is low, the wafer must be heated to about 200.degree. C. or higher to obtain a satisfactory ashing rate. If the wafer is thus heated, however, a minute amount of heavy metals and alkaline metals contained in the resist is introduced into the wafer, whereby the contamination problems of the wafer arises, as is well known in the art.
Accordingly, the development of a technique by which a satisfactory ashing rate can be obtained at a low temperature, at which contamination of the wafer does not occur, is underway.
2. Description of the Related Art
The resist ashing method of the prior art is described with reference to the device used for the downstream ashing method shown in FIG. 4.
As shown in FIG. 4, a vacuum reaction chamber 6 is connected to the vacuum pump P, the reaction gases are fed through the gas introducing inlet 3 into the plasma generation chamber 4 provided in reaction chamber 6, and are then formed into a plasma by, for example, microwaves at a frequency of, for example, 2.45 GHz, transmitted through a microwave transmission window 2 by a waveguide 1.
Among the ions, electrons, and active species formed by plasma formation, the ions and the electrons are shielded by the earthed shower head 5, and residual active species pass through the shower head 5 and flow down toward the wafer 8 on the stage 7. The active species come into contact with the wafer 8 and remove the resist film, comprising an organic material (not shown) coated on the wafer 8, by ashing.
In the prior art, the downstream ashing method usually practiced uses the device shown in FIG. 4, in which is used the method by which a gas composed mainly of oxygen is introduced, as the gas for ashing, through the gas introducing inlet 3, formed into a plasma, and subjected to a downstream to bring the active species of oxygen atoms into contact with the wafer 8.
According to this method, since the activation energy during the ashing is as high as 0.52 eV, the influence on the ashing rate of the temperature (i.e., temperature dependency) is great, and thus the reproducibility and controllability of the ashing are poor. Further, when the stage temperature is 180.degree. C., the ashing rate is as low as 0.2 .mu.m/min and therefore, to obtaining the minimum limit of an about 0.5 .mu.m/min. of ashing rate required for practical application, the wafer 8 must be heated to about 200.degree. C. or higher, and accordingly, the contamination problems of the wafer 8 arises.
Accordingly, methods which enable ashing at a lower temperature than that used in the downstream method by using a gas composed mainly of oxygen, have been proposed and are described as follows.
(a) Downstream ashing method using oxygen (O.sub.2) and water (H.sub.2 O):
This method uses the device shown in FIG. 4, which performs ashing by introducing O.sub.2 and H.sub.2 O through the gas introducing inlet 3 to form the same into a plasma, which is then subjected to a downstream to bring active species of oxygen atoms formed from oxygen, oxygen atoms, and hydrogen atoms and OH formed from H.sub.2 O, into contact with the wafer 8.
In this method, the relationship between the ratio of H.sub.2 O to the total amount of the gas mixture of O.sub.2 and H.sub.2 O is shown in FIG. 6. It should be noted that the stage temperature is 180.degree. C., and the flow rate of the gas mixture of O.sub.2 and H.sub.2 O is 1 liter/min.
As seen from the FIG. 6, when the ratio of H.sub.2 O is increased, the ashing rate reaches a maximum value of 0.35 .mu.m/min., which is about 2-fold that of the ashing method using a gas composed mainly of oxygen, when the content of H.sub.2 O is 30% to 40%. Even when the ratio of H.sub.2 O is further increased, the ashing rate is not substantially lowered.
This is considered to be because the oxygen atoms formed from H.sub.2 O and other active species participate in the ashing, together with the oxygen atoms formed from oxygen.
Also, the activation energy is less, at 0.39 eV, compared with the ashing method using a gas composed mainly of oxygen. This is considered to be due to a lowering of the activation energy by OH formed primarily from H.sub.2 O.
As the result, in addition to enabling ashing at a temperature lower than that used in the ashing method using a gas composed mainly of oxygen, the temperature dependency is reduced, and thus the reproducibility and controllability are improved. To obtain a practical ashing rate, however, the wafer 8 must be heated to about 200.degree. C. or higher, and thus the contamination problems of the wafer still remains.
Next, in the method using a gas composed mainly of oxygen, it is known that a satisfactory rate can be obtained at a low temperature by an addition of a fluorine gas, as described below:
(b) Downstream ashing method using a gas containing oxygen and a halogen:
It is known in the art that the ashing rate of 1 .mu.m/min. or higher, which is about 5-fold higher than that of the method using a gas composed mainly of oxygen, can be obtained when about 10% to 15% of a gas containing a small amount of halogen, for example, carbon tetrafluoride (CF.sub.4) in oxygen, is added to the plasma by using the device shown in FIG. 4.
This is because the gas containing a halogen promotes the dissociation of oxygen into oxygen atoms, when the gas containing oxygen and a halogen is formed into a plasma.
Further, when even a small amount of a gas containing fluorine, as a halogen, is added, and the active species of the fluorine come into contact with the wafer 8, the activation energy during ashing is remarkably lowered to about 0.05 eV, compared with the method using only oxygen (V. Vukanovic et al., J. vac. Sci. Technol., B6 (1), Jan/Feb 1988 pp. 66, J. M. Cook and Brent W. Benson, J. Electrochem. Soc. Vol., 130,No. 12, December, 1983, pp. 2459).
In this method, the active species of oxygen atoms are increased, to improve the ashing rate, and at the same time, ashing at room temperature is possible due to a lowering of the activation energy by the action of fluorine, and further, an additional advantage is gained in that the temperature dependency is greatly reduced. Nevertheless, when the fluorine atoms reach the SiO.sub.2 surface, a drawback arises in that the substrate layer of, for example, SiO.sub.2, may be etched during the ashing process.
As a method which provides a satisfactory ashing rate even at room temperature, the method of performing ashing by using a gas containing mainly fluorine is known. This method is described below.
(c) Downstream ashing method by addition of water (H.sub.2 O) during downstream of nitrogen trifluoride (NF.sub.3):
As shown in FIG. 5, a downstream ashing device which is an improvement of the device shown in FIG. 4 is used
This method comprises injecting NF.sub.3 through the gas introducing inlet 3 to form a plasma, and then subjecting the active species of fluorine atoms to a downstream through the shower plate 5. During the downstream from the addition inlet 9, H.sub.2 O is added in an amount smaller than the amount of NF.sub.3 injected, whereby the chemical reaction with fluorine and H.sub.2 O, as shown below, occurs EQU 2 F+H.sub.2 O.fwdarw.2HF+O
to generate the oxygen atoms necessary for ashing. Also, residual fluorine atoms which have not undergone the chemical reaction withdraw H from the C--H bond on the resist surface, thereby substituting for the H (H. Okano et al. The Electrochem. Soc. Spring Meeting, Atlanta, May 15-20, 1988).
According to this method, the ashing rate is improved by the oxygen atoms formed by the chemical reaction between the H.sub.2 O and fluorine atoms, and the residual fluorine atoms, which have not undergone the chemical reaction. Further, the activation energy is lowered by a substitution of H from the C--H bond on the resist surface with fluorine atom, and accordingly, in addition to enabling ashing at room temperature, an advantage is gained in that the temperature dependency is greatly reduced.
Nevertheless, because fluorine atoms reach the surface of the wafer 8 the problem of an etching of the substrate layer such as SiO.sub.2 arises, and further, because oxygen atoms are obtained by an exothermic reaction between fluorine atoms and H.sub.2 O, the wafer surface is heated by excess heat radiated therefrom, whereby the contamination problems of the wafer 8 with, for example, heavy metals, arises.
In the prior art methods described above, a gas composed mainly of oxygen is used, and in the downstream ashing method with oxygen and water of (a), the wafer temperature must be raised to 200.degree. C. or higher, to obtain a practical ashing rate, and thus the problem of wafer contamination arises.
In the methods of using a halo-containing gas, namely (b) the method of using a gas containing oxygen and a halogen, and (c) the method of adding water during the downstream of nitrogen trifluoride (NF.sub.3), because fluorine, which a gas containing a halogen, is used to obtain a practical ashing rate even at room temperature, a problem arises in that the substrate layer of for example, SiO.sub.2, is etched.
Further, in the downstream ashing method of (c), because the chemical reaction between water and fluorine atoms is an exothermic reaction, and therefore, the wafer surface is excessively heated, and thus the contamination problems of the wafer can arise.
As described above, according to the methods of the prior art, it is impossible to obtain a satisfactory ashing rate at a low temperature without etching the substrate layer, and without generating a contamination of the wafer.
Recently, the present inventors briefly reported a possibility of a downstream ashing method using O.sub.2, CF.sub.4, and H.sub.2 O as an ashing gas in Extended Abstracts of the 49th Autumn Meetings, 1988; The Japan Society of Applied Physics page 553 (October, 1988). This method is disclosed in a copending U.S. patent application Ser. No. 361178 filed June 5, 1989.